Properties of the Stimulated Electromagnetic Emissions During the Inclined High‐Frequency Pumping of the Ionosphere Near the Fourth Electron Gyroharmonic at the High‐Frequency Active Auroral Research Program Facility.

Many phenomena in the high‐frequency pumped ionosphere exhibit dependence on the pump beam incident angle α. This motivates a systematic study of the α dependence of the stimulated electromagnetic emission (SEE), particularly near electron gyroharmonics. We report the first observations of stationary SEE spectra for α ranging from −28° (north) to 28° (south) at three receiving sites, for the pump frequency (f0) sweeping near the fourth gyroharmonic (4fc). The following is established: (i) For pumping near the magnetic zenith (α = 7°, 14°, 21°), when existent dynamic broad upshifted maximum in the SEE spectrum indicates that artificial ionization layers are excited, suppression of the downshifted maximum at f0 ≈ 4fс is weakest, and 4fс increases. (ii) Weaker similar effects occur for α = −14° (a condition called "mirror magnetic zenith"). (iii) For northern pump beam inclinations, the SEE intensity decreases (in comparison with southern inclinations and vertical), most strongly at the southernmost receiving site. Plain Language Summary: Many phenomena in the high‐frequency (HF)‐pumped upper ionosphere, such as artificial airglow, descending artificial ionization layers, pump‐induced plasma irregularities, and HF plasma modes, exhibit dependence on the pump beam incident angle with respect to the ionosphere. This motivates the systematic study of angle dependence of the stimulated electromagnetic emission (secondary radiation from the ionosphere) appearing due to plasma mode conversion into electromagnetic ones, particularly near electron gyroharmonics. We report the first observations of secondary radiation spectra as dependent on mismatch between the pump frequency and fourth gyroharmonic, for incident angle varying from −28° (north) to 28° (south) at different receiving sites. The results obtained indicate that the interaction between high‐power high‐frequency pump wave and plasma modes in the ionospheric plasma takes place in a wide range of pump beam inclination angles; the interaction is stronger for southern inclinations. Newly discovered emission peculiarities, such as the emission suppression decrease at the fourth gyroresonance and energy redistribution between different emission spectral components (both for pumping to the magnetic zenith, 14°, when artificial ionization occurs) and similar but weaker effects for "mirror magnetic zenith" (−14°), allow deeper understanding of the physical processes occurring in the HF‐pumped upper ionosphere. Key Points: Properties of the stimulated electromagnetic emission are studied for different pump wave beam inclinations near the fourth gyroharmonicFor pumping at magnetic zenith, a correlation between emission spectral lines and artificial ionization layers is obtainedFor northern pump beam inclinations, the emission intensity decreases, most strongly at the southernmost receiving site [ABSTRACT FROM AUTHOR]